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# What does it mean when there is zero tension in the coupling watch

1. For part iii) What does it mean when there is zero tension in the coupling?
Also, how does the car and trailer have the same acceleration if they are no longer connected? The diagrams in the mark scheme for iii) have confused me quite a bit.Just need help with part iii)
Full question and markscheme is below:

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2. (Original post by h26)
For part iii) What does it mean when there is zero tension in the coupling?
Also, how does the car and trailer have the same acceleration if they are no longer connected? The diagrams in the mark scheme for iii) have confused me quite a bit.Just need help with part iii)
Full question and markscheme is below:
Zero tension in the coupling means that the car is not pulling the trailer and the trailer is not pushing the car. The resistive forces are such that, even if the car and trailer were uncoupled, they would continue to accelerate (decelerate in this case) at the same rate as if they were coupled. That is why the mark scheme has considered the forces acting on the car and trailer separately, even though there is no indication in the question that they have actually been uncoupled.
3. (Original post by old_engineer)
Zero tension in the coupling means that the car is not pulling the trailer and the trailer is not pushing the car. The resistive forces are such that, even if the car and trailer were uncoupled, they would continue to accelerate (decelerate in this case) at the same rate as if they were coupled. That is why the mark scheme has considered the forces acting on the car and trailer separately, even though there is no indication in the question that they have actually been uncoupled.
Thank you very much. In what kind of example would the car and trailer have a different acceleration ? Also, how do you know the resistance forces acting on the car and the trailer would allow the car and trailer to have the same acceleration? The resistances forces are different for the car and the trailer and the mass of the car and trailer are different too so surely acceleration is different if we consider F=ma for each?
4. (Original post by h26)
Thank you very much. In what kind of example would the car and trailer have a different acceleration ?
You may come across questions where a car is towing a trailer uphill and the trailer becomes unhitched while the car continues to drive uphill.

(Original post by h26)
Also, how do you know the resistance forces acting on the car and the trailer would allow the car and trailer to have the same acceleration?
The car and the trailer are coupled together but neither is exerting any force on the other. The fact that the car and trailer are not influencing each other's motion allows us to assume that they would both continue with the same motion if the coupling was disconnected.

(Original post by h26)
The resistances forces are different for the car and the trailer and the mass of the car and trailer are different too so surely acceleration is different if we consider F=ma for each?
Not necessarily. a = F/m so a will be the same if the ratio F/m is the same. We have already concluded that a is the same for car and trailer, so it follows that F/m must be the same too.
5. (Original post by old_engineer)
You may come across questions where a car is towing a trailer uphill and the trailer becomes unhitched while the car continues to drive uphill.

The car and the trailer are coupled together but neither is exerting any force on the other. The fact that the car and trailer are not influencing each other's motion allows us to assume that they would both continue with the same motion if the coupling was disconnected.

Not necessarily. a = F/m so a will be the same if the ratio F/m is the same. We have already concluded that a is the same for car and trailer, so it follows that F/m must be the same too.
Thank you very much!!

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